BUILTIN.DF Not working on transaction.employee

Anyone have a clue as to why BUILTIN.DF is not returning the display value for the salesrep field on an Invoice? It is only retuning the internalid of the employee record, not the name. I added the LEFT JOIN just to see the side by side result.

SELECT t.tranid,
BUILTIN.DF(t.employee) as sales_rep,
       employee.firstname,
       employee.lastname
FROM transaction t
         left join employee on t.employee = employee.id
WHERE t.tranid = 'xxxxxxxx'

SELECT BUILTIN.DF(employee) FROM transaction is working in the account where I’ve just run it.